Iran arrests 30 at yoga class over 'inappropriate' outfits
They were detained at a private residence in Gorgan, a city in the north of the country, Tasnim news agency said.
The instructor, who was also arrested, did not have a licence to run the class and had advertised the event on Instagram, the report said.
Local justice department official Massoud Soleimani said participants of the class were dressed in "inappropriate outfits", and had "behaved inappropriately" by taking part in the mixed class.
Mixed-gender sports activities are banned in Iran and yoga instructors must have a license to teach it in single-sex sessions.
Following the 1979 Islamic revolution, compulsory hijab was enforced throughout Iran with women forced to dress modestly and cover themselves with the Islamic veil.
In 2017, an Iranian female bodybuilder was imprisoned for posting gym selfies showing her bare arms and biceps.
The woman had been accused of posting "un-Islamic" and "nude" images of herself on social media - breaking the Islamic Republic's strict rules on 'revealing' clothing.
Women in Iran are also banned from attending men's football matches in the country. In 2017, eight women disguised themselves as men to go to a football match in Tehran, defying the ban.
Follow us on Twitter: @The_NewArab